1. Field of the Invention.
This invention relates to a flame detection circuit. More particularly, this invention relates to an amplifier in a flame detection circuit including a hot refractory detection circuit which substantially eliminates a false flame signal caused by hot refractory shimmering.
2. Description of the Prior Art.
There are many applications for large boilers and burners which use flame heating. In all of these applications, it is desirable to be able to detect whether the flame is present in order to control the flow of fuel to the burner or boiler. There are several ways of detecting the presence of a flame. Generally, a flame detector, or photocell, is used to detect the presence of a flame. An amplifier is used to amplify the signal emitted by the flame detector. This amplified signal is monitored by a burner control system which controls fuel pumping mechanisms based on, among other things, the amplified signal. Flame detectors include ultraviolet radiation detectors, visible light detectors, and infrared sensors which detect the flickering of the flame.
Previous sensing methods have not been fully satisfactory in dealing with a problem known as "hot refractory." Typically, in a burner or boiler, the fire is confined in a chamber which is lined with refractory material. When the burner has been on for a long time, the refractory material begins to glow red. Even if the flame goes out, the red glow of the refractory material does not immediately disappear. Therefore, the visible light sensors do not accurately represent whether the flame is present. Additionally, air currents passing over the hot refractory material can cause a shimmering effect which deceives an infrared flame flicker sensor (IR sensor) into producing a flame signal which erroneously represents that a flame is still present. As long as the flame sensor does not detect that the flame has gone out (a flame-out condition), the burner control system will continue to supply fuel to the burner. If there is in fact no flame, the congregation of fuel in the burner area may result in a very severe explosion.
Because of the severe consequences resulting from an error in the flame detection circuitry, an important and desirable characteristic of the flame detection circuit is reliability. Also, the circuitry must be able to detect the difference between an actual flame and hot refractory shimmering.
To achieve reliability, some method must be implemented for testing the detection circuitry. One method for testing flame detection circuitry is disclosed in the Rowell U.S. Pat. No. 3,202,976. This method involves feeding back the output of the flame detection circuitry to the input in such a way that makes the input disappear. When the input disappears, of course, the output disappears and consequently, through feedback, the input reappears. Therefore, a cyclical condition is created at the output which can only arise if there truly is an output to begin with. The frequency of the cyclical condition depends on the time constants of all the various circuits in the amplifier. However, the technique used to achieve reliability in the Rowell patent does not deal with the problem of an erroneous flame signal caused by hot refractory shimmering Moreover, that technique is not readily applicable to a flame detector based on the use of an IR sensor to detect flame flicker. This is because feedback of the output to the input to achieve the cyclical condition at the output when a true input is present can, by itself, cause a cyclical output.
A desirable characteristic of any test method is that the test should encompass all of the hardware in the flame detection circuit, if possible This is very difficult to achieve and there is a continuing need for these test methods.
Noise rejection is also very important and desirable in a flame detection circuit. Techniques must be employed throughout the circuitry to maximize the signal-to-noise ratio. Also, since the burners require some type of ignition, the detection circuitry should reject spark ignition noise. Similarly it is desirable that the circuit be insensitive to noise with a frequency of about 60 Hz since that noise is generally prevalent in industrial burners of this type and since the maximum flicker intensity for a gas flame occurs at approximately 12 to 14 Hz.
Some of the flame amplifiers which are presently used in flame detection circuitry reject some high frequency noise but pass frequencies as low as 2 to 3 Hz. However, a step input to the amplifier, which occurs for instance when the flame goes out, results in damped oscillation ringing through the amplifier. The frequency of the ringing depends on the time constants throughout the amplifier. Therefore, if the flame suddenly goes out, the low frequency, damped sinusoid, which occurs at approximately 2 to 3 Hz, may last long enough and be amplified enough to be mistaken as a flame-on condition by the control system. Therefore, amplifiers which pass low freqencies in the range of 2 to 3 Hz are undesirable.